This invention is generally directed to processes for charging imaging members such as photoreceptors, photoconductive imaging members and dielectric charge receivers for ionography. More specifically, in embodiments the present invention relates to processes for charging photoconductive imaging members, especially and preferably layered imaging members by ionic conduction and wherein, for example, corona charging and discharging devices together with their known disadvantages can be avoided and/or minimized. Embodiments of the present invention include a process for the ion transfer charging of photoconductive imaging members, which process comprises contacting a component, such as a liquid like water, with the surface of the imaging member; and applying a voltage to the component while moving, such as rotating the imaging member thereby enabling the transfer of ions, preferably of a single sign, such as positive or negative, from the liquid/imaging member interface to the imaging member.
The charging of photoconductive imaging members by means of corona discharge methods is known, however, a number of disadvantages are associated with these methods, such as the generation of ozone, the use of high voltages, such as from about 6,000 to about 7,000 volts, which requires the use of special insulation, maintenance of the corotron wires at added costs, low charging efficiency, the need for erase lamps and lamp shields, and the like. Since it is a health hazard, ozone is removed by passage through a filter. Corona charging generates oxides of nitrogen which desorb eventually from the corotron surfaces and eventually oxidize the transport molecule thereby adversely effecting the electrical properties of the photoreceptor. These can show up as print deletions.
Generally, the process of electrostatographic copying is initiated by placing a substantially uniform electrostatic charge on a photoreceptive member. Subsequent to this charging, imaging is accomplished by exposing a light image of an original document onto the substantially uniformly charged photoreceptive member. Exposing the charged photoreceptive member to a light image discharges the photoconductive surface thereon in areas corresponding to nonimage areas in the original document while maintaining the charge in image areas, thereby creating an electrostatic latent image of the original document on the photoreceptive member. This latent image is subsequently developed into a visible image by depositing charged developing material onto the photoreceptive member such that the developing material is attracted to the charged image areas on the photoconductive surface. Thereafter, the developing material is transferred from the photoreceptive member to a copy sheet or to some other image support substrate for creating a visible image which may be permanently affixed to the image support substrate, thereby providing a reproduction of the original document. In a final step in the process, the photoconductive surface of the photoreceptive member can be cleaned to remove any residual developing material which may be remaining on the surface thereof in preparation for successive imaging cycles.
Illustrated in copending patent application U.S. Ser. No. 176,188 filed Jan. 3, 1994, the disclosure of which is totally incorporated herein by reference, is a corona generating device and, more particularly, a reusable corona charging apparatus for use in an electrostatographic printing machine to generate a flow of ions onto an adjacent imaging surface so as to alter the electrostatic charge thereon.
The electrostatographic copying process described hereinabove is well known and is commonly used for light lens copying of an original document. Analogous processes also exist in other electrostatographic printing applications such as, for example, digital laser printing where a latent image is formed on the photoconductive surface via a modulated laser beam, or ionographic printing, and reproduction where charge is deposited on a charge retentive surface in response to electronically generated or stored images.
In addition to charging the imaging surface of an electrostatographic system prior to exposure, corona devices are used to perform a variety of other functions in the electrostatographic process. For example, corona generating devices aid in the transfer of an electrostatic toner image from a reusable photoconductive imaging member to a transfer member such as paper; the tacking and detacking of the transfer member to and from the imaging member; and the conditioning of the surface of the imaging member prior to, during, and after deposition of toner thereon to improve the quality of the electrostatographic copy produced thereby. Each of these functions can be accomplished by a separate and independent corona generating device. The relatively large number of devices within a single machine necessitates the economical use of corona generating devices.
Various types of charging devices have been used to charge or precharge the surface of a photoconductive member. Corona generating devices are used extensively, wherein a voltage of 2,000 to 10,000 volts may be applied across an electrode to produce a corona spray which imparts electrostatic charge to a surface situated in close proximity thereto. One particular corona generating device includes a single corona generating electrode strung between insulating end blocks mounted on either end of a channel formed by a U-shaped shield or a pair of spaced side shield members. The corona generating electrode is typically a highly conductive, elongated wire positioned opposite the surface to be charged. In other conventional corona generating devices, the corona generating electrode may also be in the form of a pin array. Another device, frequently selected to provide more uniform charging and to prevent overcharging, includes two or more corona generating electrodes with a control grid comprising a screen having a plurality of parallel wires or a plate having multiple apertures positioned between the corona generating electrodes and the photoconductive member. In this device, a potential having the same polarity as that applied to the corona electrodes but having a much smaller voltage magnitude, usually about a few hundred volts, is applied to the control grid to suppress the electric field between the control grid and the corona electrodes, markedly reducing the ion current flow to the photoconductive member.
Yet another type of corona generating device is described in U.S. Pat. No. 4,086,650 wherein a corona discharge electrode is coated with a relatively thick dielectric material such as glass for substantially preventing the flow of conduction current therethrough. In this device, the delivery of charge to the photoconductive member is accomplished by a displacement current or by capacitive coupling through the dielectric material. The flow of ions to the surface to be charged is regulated by means of a DC bias applied to the shield of the corona generating device. In operation, an AC potential of approximately 5,000 to 7,000 volts is applied to the coated electrode at a frequency of about 4 KHz to produce an actual corona generating current of approximately 1 to 2 milliamperes. This device has the advantage of providing a uniform charge to the photoconductive member using a charge generating device that is highly insensitive to contamination by dirt and, therefore, does not require repetitive cleaning or other maintenance requirements.
One problem associated with corona generating devices occurs in the presence of the generated corona, wherein a region of high chemical reactivity is also produced such that new chemical compounds are synthesized in the machine air. This chemical reactivity correspondingly causes a build up of chemical growth on the corona generating electrode as well as other surfaces adjacent thereto. After a prolonged period of operation, these chemical growths may degrade the performance of the corona generating device and also the entire electrostatographic machine.
Free oxygen, ozone, and other corona effluents, such as nitrogen oxide, and nitrogen oxide species, can be produced in the corona region. These nitrogen oxide species react with solid surfaces. In particular, it has been observed that these nitrogen oxide species are adsorbed by the conductive control grid, the shield, shield members and other components of the corona generating device. The adsorption of nitrogen oxide species occurs even though the corona generating device may be provided with a directed air flow during operation for removing the nitrogen oxide species as well as controlling ozone emissions. During the process of collecting ozone, directed air flow may exacerbate problems by carrying the nitrogen oxide species to an affected area of the corona generating device or even to some other machine part.
The reaction of corona generating process byproducts, such as nitrogen oxide, with the shield, the control grid, or other corona generating device components can result in corrosive buildup and deposition on the surfaces thereof. These deposits can cause problems, such as nonuniform photoreceptor charging, manifested by side-to-side density variations, or dark and light streaks in an output copy. Also, depending on environmental conditions, deposits may charge up and effectively increase the shield or screen voltage resulting in similar nonuniformity defects. Extreme cases of corrosion can lead to arcing between the corona generating electrode and the screen on the shield members.
Another problem associated with corona generating devices operating in a electrostatographic environment results from toner accumulation on the surface of the corona generating electrode as well as surfaces adjacent thereto. The spots of accumulated toner, being a dielectric in nature, tend to cause localized charge buildup on the interior surfaces of the shield which produces current nonuniformity and reduction in corona current. Localized toner accumulations on the insulating end blocks which support the wire electrode also cause sparking.
Moreover, adsorption can be a physically reversible process such that the adsorbed nitrogen oxide species are gradually desorbed when a machine is turned off for an extended period of idleness. The adsorbed and desorbed species are both nitrogenous but not necessarily the same, that is there may be a conversion of NO.sub.2 to HNO.sub.3. When the operation of the machine is resumed, a copy quality defect, commonly referred to as a parking deletion, can result wherein a line image deletion or a lower density image is formed across the width of the photoreceptor at that portion of its surface resting opposite the corona generating device during the period of idleness. It is believed that the nitrogen oxide species interact with the surface of the photoreceptor to increase the lateral conductivity thereof such that the photoreceptor cannot effectively retain a charge in image configuration. This phenomenon basically causes narrow line images to blur or to wash out so as to not be developed as a toner image.
In corona generating devices, it has been found that the material from which the components, such as the shield or control grid, are fabricated has a significant effect on the severity of parking deletions. In the prior art, stainless steel materials, such as shields, have commonly been used. Other materials, such as corrosion resistant ferrous materials which prevent the rapid oxidation of the component material and the concurrent loss of performance of the corona generator, have met with limited success, primarily due to the corrosive effect of the corona produced by the device.
In other attempts to reduce the problems associated with corona charging, considerable effort has been accomplished to reduce the adsorption of nitrogen oxides species by device components via the application of electrodag coatings to the surfaces thereof. These coatings typically include a reactive metal base such as nickel, lead, copper, zinc or mixtures thereof. These reactive metal base materials tend to absorb, or form harmless compounds with the nitrogen oxide species. However, parking deletion problems have continued due, for example, to the failure of the electrodag materials to continue to absorb or form harmless compounds with the nitrogen oxide species over time. In addition, certain components needed to address this problem are costly to fabricate.
Thus, the problem of chemical growth buildup in and around corona generating devices has been addressed by providing coating materials that are less prone to chemical attack. While adequately addressing the problem, such materials have substantially increased the cost of corona generating devices. Various forms of corona generating devices have been described for use in electrostatographic reproduction machines.
U.S. Pat. No. 4,258,258 discloses a corona generating device having a corona generating electrode supported between a pair of end block assemblies. Each end block assembly defines a space for the passage of the electrode, and nonconductive inserts for surrounding the electrodes that are seated in the spaces of the end block assemblies. The nonconductive inserts are made from a high dielectric strength material that is also resistant to a corrosive atmosphere. The inserts are easily and inexpensively replaced so as to protect the end block assemblies from the effects of high voltage applied to the corona electrode.
U.S. Pat. No. 4,585,320 discloses a corona generating device for depositing negative charge on an imaging surface carried on a conductive substrate comprising at least one elongated conductive corona discharge electrode, means to connect the electrode to a corona generating potential source, at least one element adjacent the corona discharge electrode capable of adsorbing nitrogen oxide species once the corona generating electrode is energized and capable of desorbing nitrogen oxide species once that electrode is not energized, the element being plated with a substantially continuous layer of lead to neutralize the nitrogen oxide species when generated. In a preferred embodiment, the corona discharge electrode comprises a thin wire coated at least in the discharge area with a dielectric material and at least one element comprising a conductive shield, and an insulating housing having two adjacent sides to define the longitudinal opening to permit ions from the electrode to be directed toward a surface to be charged, both the shield and the two sides of the housing being plated with a continuous thin layer of lead.
U.S. Pat. No. 4,585,322 discloses a corona generating device similar to that discussed in previously referenced and described U.S. Pat. No. 4,505,320, wherein the element adjacent the corona discharge electrode capable of adsorbing nitrogen oxide species once the corona generating electrode is energized and capable of desorbing nitrogen oxide species once that electrode is not energized is coated with a substantially continuous thin dehydrated alkaline film of an alkali silicate to neutralize the nitrogen oxide species when generated.
U.S. Pat. No. 4,585,323 discloses a corona generating device similar to that described in above referenced and described U.S. Pat. No 4,585,320 and U.S. Pat. No 4,585,322, wherein the element adjacent the corona discharge electrode capable of adsorbing nitrogen oxide species once the corona generating electrode is energized and capable of desorbing nitrogen oxide species once that electrode is not energized is coated with a substantially continuous thin layer of a paint containing reactive metal particles which will combine with the nitrogen oxide species, the reactive metal being present in the paint in an amount sufficient to neutralize the nitrogen oxide species when generated. Preferably, the reactive metal particles comprise lead, copper, nickel, gold, silver, zinc or mixtures thereof. Also of interest are U.S. Pat. Nos. 2,987,660, see for example column 2, lines 50 to 68, column 3, lines 49 to 70, and specifically column 3, lines 59 to 61, wherein water is mentioned as a conductive liquid; 3,394,002; and 2,904,431.
Generally, layered photoresponsive imaging members are described in a number of U.S. patents, such as U.S. Pat. No. 4,265,900, the disclosure of which is totally incorporated herein by reference, wherein there is illustrated an imaging member comprised of a photogenerating layer, and an aryl amine hole transport layer. Examples of photogenerating layer components include trigonal selenium, metal phthalocyanines, vanadyl phthalocyanines, and metal free phthalocyanines. Additionally, there is described in U.S. Pat. No. 3,121,006 a composite xerographic photoconductive member comprised of finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. The binder materials disclosed in the '006 patent comprise a material which is incapable of transporting for any significant distance injected charge carriers generated by the photoconductive particles.
Photoresponsive imaging members with squaraine photogenerating pigments are also known, reference U.S. Pat. No. 4,415,639. In this patent, there is illustrated a photoresponsive imaging member with a substrate, a hole blocking layer, an optional adhesive interface layer, an organic photogenerating layer, a photoconductive composition capable of enhancing or reducing the intrinsic properties of the photogenerating layer, and a hole transport layer. As photoconductive compositions for the aforementioned member, there can be selected various squaraine pigments, including hydroxy squaraine compositions.
Moreover, there are disclosed in U.S. Pat. No. 4,419,427 electrographic recording mediums with a photosemiconductive double layer comprised of a first layer containing charge carrier perylene diimide dyes, and a second layer with one or more compounds which are charge transporting materials when exposed to light, reference the disclosure in column 2, beginning at line 20.
The disclosures of each of the above patents are totally incorporated herein by reference.